Method and apparatus for identifying metals



Feb. 9, 1960 E. H. GREENBERG ET AL 2,924,771

METHOD AND APPARATUS FOR IDENTIFYING METALS Filed March 4, 1955 3 Sheets-Sheet l 1 Fig.1 2

.50 INVENTORS ELMEI? H. GREENBERG 49 4 7 WILLIAM B. GREENBERG A77 ENE) 1960 E. H. GREENBERG ET AL 2,924,771

METHOD AND APPARATUS FOR IDENTIFYING METALS Filed March 4, 1955 3 Sheets-Sheet 2 CONTkOLLEZ:

Fig.5

WILLIAM B. GREENBERG Feb. 9, 1960 E. H. GREENBERGET AL 2,

METHOD AND APPARATUS FOR IDENTIFYING METALS Filed March 4, 1955 5 Sheets-Sheet 3 PRDBE PROBE V TEMPERATURE P7,; 1 E 1 7c- 72; NARROW AAIIGLE PROBE F 313; r Q TEMP! I T on/v k b J F5. /4A u MFFUSIWTY g SAMPLE J E 7;, ZZ fi Q 72 Q Low DIFFUS/V/TY SAMPLE Too F/e'EQuE/vr REAP/N65 Lu w/m LARGE AREA E Tc c @575 k mum TEMP SAMPLE s T Hjj/ F51. /2 5 /6 a M INVEN TOR. EZMER H. GREENBERG WILLIAM B. GREENBERG ATT NE? United States Patent METHOD AND APPARATUS FOR IDENTIFYING METALS Elmer H. Greenberg, Philadelphia, and William B. Greenberg, Wynnevvood, Pa.

This invention relates generally to improvements in apparatus and methods for the non-destructive identification of metals, metalloids, and mixtures, compounds and alloys thereof, including combinations thereof with nonmetallics, and intended hereinafter to be comprehended in the term metals. The invention is more particularly directed to such apparatus and methods as employ the thermoelectric effect of a specimen being tested or identified.

The particular structural embodiment of the present invention which is illustrated in the drawings and will be described hereinafter in detail, is generally of the type having a pair of electrically connected metallic contact members adapted to be maintained at different temperatures, and selectively engageable with a metal specimen to produce a thermoelectric voltage between the contact members.

As is well known in the field of metallurgy, accurate measurements of thermoelectric effects provide some indication of identity of metals. However, accurate deter minations of thermoelectric effect have heretofore required the external application of heat to a junction of metal wires or rods until an equilibrium condition is reached while maintaining cold junctions at the other ends. This of course consumes considerable time and requires that the contacting pieces be properly shaped, making the procedure slow and impracticable under actual conditions of use. While there have in the past been metals identifying devices of another type having a heated contact member and one at room temperature engageable with spaced points of a specimen to produce a thermoelectric voltage, such devices have beenincapable of accurate results in practice except by changing the contact members until a zero voltage is obtained. This of course determines by comparison that the specimen is the same material as the contact member, but is a cumbersome and laborious procedure, and often impossible with the great number of metals used today. Upon engagement of the different temperature contact members with a specimen, the hotter of the members will conduct heat to the specimen to produce a hot junction, the contact temperature of which varies as a function of time and the heat transfer characteristics of the hot member and specimen. In prior devices therefore, accurate and repeatable readings were impossible.

While absolute steadytemperature may not he arrived at, at least with hot junction temperatures sufiiciently high to produce distinguishable readings without complicated voltage reading devices, ithas been found possible by the instant invention to provide the heat transfer characteristics adapted to produce a hot junction contact temperature which is obtained substantially instantane'ously upon engagement and remains substantially constant for a long period thereafter. Stated otherwise, the present invention contemplates the provisions of a metals identifying method and apparatus for use therein, wherein a pair of electrically connected contact members 2 at different temperatures may be engaged with an unknown specimen to substantially instantaneously and for a reasonable period produce a relatively constant thermoelectric voltage which is readily repeatable with the same specimen and others of the same properties. 7 It is another object of the present invention to provide a method and apparatus for identifying metals, wherein the indications afforded by the. various metals are spread apart over a relatively wide range to simplify and facilitate identification. More particularly, theinstant method and apparatus are adapted to provide relatively high hot junction contact temperatures, without excessive hotcontact member temperature, to space apart the thermoelectric voltage readings produced by the various metals.

However, there are instances where the thermoelectric voltage produced by different'specimens may be undistinguishable, as when the allowable tolerances in manufacture of the specimens result in the readings for an alloy being spread over a range or a band and two such thermoelectric voltage ranges or bands overlap. Of course the increased accuracy of the present invention may aid in differentiating as it will reduce the bands to minimum widths. However, the present inventionincludes a novel method and apparatus for changing the parameters of heat transfer while remaining within the limiting conditions necessary to maintain a constant hot junction contact temperature, and thus produce widely difierent thermoelectric voltages to differentiate between materials otherwise producing substantially the same ther rnoelectric readings with the original parameters.

it is a further object of the present invention to provide a metals identifying device having the advantageous characteristics mentioned hereinbefore, which is extremely simple in construction and operation, foolproof and durable in use, and which can be manufactured and sold at a reasonable cost. 1

Other objects of the present invention will become apparent by reading thefollowing specification and referring to the accompanying drawings, which form a material part of this disclosure.

The invention accordingly consists in the features of construction, and combinations and arrangements of ele-' ments and method steps, which will be exemplified in the following description, and of which the scope will be indicated by the appended claims. I

In the drawings: I

Figure l is a side view showing a device constructed in accordance with the present invention;

Fig. 2 is a transverse sectional view taken substantially along the line 2-.-.-2 of Fig. 1;

Fig. 3 is a longitudinal sectional view taken substantially along the line 3-3 of Fig. 1;

Fig. 4 is a partial, perspective view showing the device of Fig. 1, with parts, broken away and exploded for purposes of clarity and understanding;

Fig. 5 is an electrical schematic of the device in use;

Fig. 6 is a schematic representation showing a slightly modified form of the electrical arrangement of the present invention.

Figs. 7-12 are diagrammatic representations of heat flow in elements of different shapes and thermal charaee teristics; and

Figs. 13 and 13A show a hot junction and graphic representation of temperature gradients in the junction with different probe angles;

Figs. 14 and 14A show a hot junction and a graphic representation of. temperature gradients in, the junction with different probe diffusivities;

Figs. 15 and 15A show a hot junction and a graphic representation of temperature gradients. in the junction with different sample diffusivities;

Figs. 16 and, 16A show a hot junction and a graphic Patented Feb. 9, 1950 representation of temperature gradients in the junction upon repeated contacts with a probe of too large a contact area.

While many years have passed since the first teaching of metals testing devices employed a pair of contact members at different temperatures engageable with a specimen to produce a thermoelectric voltage, and although their potential value for quick, simple and non-destructive metals identification has been long appreciated, such devices with their variable and fluctuating readings have found no commercial acceptance. This is no doubt due to the knowledge that apoor testing instrument, giving erroneous results may be extremely costly in manufacturing parts of wrong material.

While the physical principles and theory underlying operation of the instant device, which result in a substantially constant hot junction contact temperature and hence steady readings, are based on empirical rationale, substantiated by exhaustive experimentation, they will be presented as an aid to understanding the invention.

In thermocouples, in general, the temperature at the junction of the two dissimilar metals is that temperature common to both metals adjacent to the area; of contact, and the thermal conductivities and dilfusivities of the metals are not involved. In the instant device for speed and convenience in operation, one of the contact members is heated; and, by diffusion and conduction heat flows into the specimen through the area of contact therebetween. The temperature atthe hot junction area, hereinafter designated'T and the resulting thermoelectric voltage, depend not only on the relative thermoelectric effects, but also on the relative thermal conductivities and diffusivities of the contact members and specimen. For simplicity in terminology, thermal conductivity and diffusivity will be abbreviated hereinafter as T.C. and D. The thermoelectric voltage produced in the instant device is never that of mere thermoelectric power of the metals involved in the probe and sample, but is substantially affected by the thermal characteristics of the hot junction member and sample or specimen. For example, T can vary from a small fraction of the temperature of the hot junction contact member or block, T from which the heat flows into thehot junction member, up

to but never equal to T and depends on the T.C. and

D. of the contact member or probe material relative to that of the specimen. Other important factors found to affect .T,,, as will appear in greater detail hereinafter, are the shape and size of the hot junction contact member or probe. Forv every probe of selected T.C. and D. at a predetermined and controlled temperature above room temperature T,, a value of T can be obtained for any sample of fixed chemical analysis and hardness condition. If the hardness of the sample remains constant or if it causes very little change in T.C. and D. of the sample, the reading is found to be reproducible when the shape and size of the probe point are properly proportioned and the proper probe metal is used. Otherwise the reading will not duplicate or be steady during reasonable reading interval. The reading is always greatly affected by the T.C. and D. of the piece or sample and these effects very frequently cause greater changes in net readings or indicated voltages than the differences caused by thermoelectric power differences in specimens. We fully appreciate and utilize these interacting conditions in our method and device. 7

With a given probe of fixed T.C. and D. we have found that two samples of similar thermoelectric effect can give different readings either because they are of different hardness and resulting different T.C. and D. or because they are of different chemical analyses and basically different T.C. and D. but by coincidence of same thermoelectric effect. Also two samples each of both different thermoelectric effect and T.C. and D. characteristics can have the same reading by coincidence against a given probe. Changing to a probe of different thermoelectric effect but almost the same T.C. and D. will still give closely similar readings as they will both be displaced almost equally on a voltmeter by an amount related to the differences between the thermoelectric effects of the two probes. We havefound that by using two probes of noticeably different T.C. and D. irrespectiveof their thermoelectric effects, we can get different readings for two samples only slightly different in T.C. and D. against one of the probes even if they read alike against the other. We have also found that for the sake of simplicity of operation and reading it is advantageous to make both probes of materials with thermoelectric effects close to each other and close to that of copper which is used in the wiring connections. Since we make use of two probes in one heated block, quick readings with both can be obtained, especially if the base of the readings is not disturbed by using a probe of much difierent thermoelectric effect, which could involve a change in sensitivity scale or direction of reading. Probes of thermoelectric effect materially different from copper are found to give cold junction errors.

It is rarely possible to resolve ambiguities by using two probes of much different thermoelectric power and close thermal diifusivities. Only if the samples involved are very much different in thermal diffusivity, will the readings diifer on one of the probes. The use of different thermoelectric probes will only move both readings on the scale similarly. I

The controlling factors for obtaining standardized reproducible readings reside largely in the control of the heat gradients formed in the heated probe and test piece on both sides of the hot junction contact area. When contact is made there is instantly a lowering of temperature on-the probe side of the junction area and 2 raising of temperature on the specimen side of the area. As contact is continued the cooling of the probe goes on further, and further back towards the main body of the heated mass of the block and at the same time heat diffuses out radially into the sample piece. We have discovered that this contact temperature will always have to bethe same for given materials in the probe and sample and remain constant during contact'for a period of time sufiicient to take and perhaps verify a reading; or, the readings will not be repeatable and will not be steady during the reading period.

The stability of contact temperature referred to herein is of an accuracy consistent with the overall accuracy of the entire device which in, turn is controlled by the various portions of the same. This overall accuracy in turn must be kept to a degree consistent with the accuracy or range of the nominal. sample analyses and physical conditions.

Although the heat transfer conditions are not steady during the period of contact, the quantity of heat flowing through the contact area decreasing with time, we have found that by properly selecting the shape and size of the probe near the area of contact, the latter also being properly selected, the temperature at the contact area can be kept constant to a practical degree.

Before studying the development of our formula for these parameters, it is advantageous to define thermal conductivity andthermal diifusivity and especially the latter. Thermal conductivity 'is the number of B.t.u. of heat flowing per hour in the material, at right angles to two planes with the area of each equal to one square foot, with one foot of space between, and with a difference in temperature of one deg. Fahr. Thermal diffusivity is thermal conductivity divided by specific heat times density, which has units of sq. ft./hr. Specific heat is the amount of heat in B.t.u. necessary to raise a pound of material one deg. F. Density is the number of pounds of material per cu. ft. Thermal conductivity is involved solely in steady heat; flow, while thermal conductivityand thermal diffusivity, each .to various degrees are involved in unsteady heat flow where a hot piece is put in contact with a cold piece as in our situation.

With time, diffusivity of heat out of the probe, through the area of contact and continuously deeper into the sample proceeds at a lower rate as the heat has to travel further in both the probe and the piece. Also with time, the conditions of the metal of the probe and piece adjacent to the area of contact become closer and closer to a steady condition involving thermal conductivity more and more. But, dilfusivity is still going on at its relatively important rate. For the sake of simplicity, we will call the involved combined characteristics of a metal related to heat flow under the conditions we are interested in as a combined thermal conductivity and diffusivity characteristic even though each effect plays a difierent part in the flow and to a different degree and perhaps even as a difierent power.

The following exposition is necessary to aid visualization. If a heated probe block, see Fig. 7, of given T.C. and D. and at temperature T is placed on a test sample block with similar faces in contact; and if the T.C. and D. of the sample block is the same as that of the probe, the depths of the heat gradient out of the probe by cooling would be the same as that into the sample by heating.

.This would be so instantly after contact and for a considerable time thereafter. The temperature T at the contact area would be half that of the T at the hot end of the block of the probe.

If the T.C. and D. of the sample was a fraction of the probe, say about half, T would instantly be adjusted to a point so that about twice as much volume would be cooled in the probe as would be heated in the sample, see Fig. 8. The total quantity of heat passing from the heated block through the contact area is of course equal to that passing into the sample. Assuming the two pieces to be of the same specific heat and density, then;

% X 1 volume Solving: T /sT and would remain constant with time.

If the samples were all of the same cross section as the probe and the contact faces, similar, good tests could be obtained this way. If the T.C. and D. of a sample piece is greater than that of the probe, T could be less than /2 and even down to a very small "fraction. With a very high T.C. and D. for the probe and a very low one for the sample the fraction could almost be to l, but of course, never equal to it.

If the test piece had no faces as small as the probe face and the probe was contacted against a larger fiat face, then T would not remain constant with time, see Fig. 9. It would be just a little lower than above at first contact, but would get noticeably lower as contact con tinues because the flow path into the sample would be of increasingly larger volume than above. Diffusion of heat out of the probe would be by increasingly longer cylinders but in the sample by means of larger and larger cones or hemispheres. Only at the very first instant would the flow be approximately cylindrical. The volume of the probe cylinder would increase only by length while the volume of a wide angle cone or hemisphere of the sample is related to the 3d power of the increased length of the difiused path and heat gradient.

If the contact area is large, the test piece is heated up considerably and on a check reading the continuously heated cylinder will give a higher T and an erroneously high reading. Large contact areas are subject to a variable, rocking contact, see Fig. 10, which further complicates and affects the balancing temperature T as the relationships of the flow paths on each side of the contact area change.

Making the cylinder smaller would serve towards more stabilization, see Fig. 11 However, T would be very low at all times an it uld tak a oss is: t

X2 volumes is equal to 6 cooled probe to be replenished with heat to-full T tern: perature. If a successive contact ismade with a probe that has not come back to full T temperature the next reading would be in error as the new T would be lower than it should be.

A cylindrical probe, Fig. 12, with a rounded end would also give a variable T with time. At the first small time interval, the flow would be as between two cylinders, and then for a short time would change to conical or hemiw spherical flow in both, and shortly after it would become conical flow, less than hemispherical, in the probe while remaining conical or hemispherical degrees) in.the sample. This would continue for a short period with an increasingly narrowing angle for the probe cones untila cylindrical flow would be established in the same.

We have discovered that if T is to remain constant as the flow paths in both pieces increase in Volume, the complicated relationships of the different path cross.- sections and lengths and T.C. and D.s on each side of the contact area will have to remain the same.

The heat flow path into the sample piece, which usually has at least one fairly flat area for contacting the probe, can be assumed as widely conical (averaging 180 degrees or somewhat less). We have discovered that the resulting flow in the volume of the probe Will also have tobe conical to maintain a constant T A pyramid will also give fairly good results.

The volume of'a hemisphere is 2.09R while that of a spherical segment is 2.09R h or %11'R h. But it is related to R by the cone angle which is a constant C,,. For a given angle C is approximately equal to h divided by R so h is equal to R C and volume is equal to 2.09R .C

We will assume R and R to be lengths of the heat gradient paths in the probe and sample respectively; T temperature of the probe block above room temperature, the latter assumed to be the temperature of the sample; T contact temperature above room temperature; sample or room temperature is equal to 0; SH and 8H,, specific heats of the probe and sample; and D and D densities of the probe and sample metals. Since we prefer the use of a small area as explained herein, the efiect of truncating the cone will be small and will be disregarded for the sake of practicability. This is justified by test. It has also been corroborated by experimentation that radiation losses may be ignored Without substantial loss of accuracy. We also assume that the temperature gradients are linear with distance, and any resultant error does not affect the use of the formulae for practical inspection purposes.

Up to any time of contact, the heat quantities flowing to cool the probe and heat the sample are equal, therefore g-T X Probe volume cooled average S H 37 X D,,

=% X sample volume heated average SH, D,

The relative lengths, R and R and the relative volumes affected are tied by an overall relationship tothe T.C. and D.s of the probe and sample so that T remains CQH? stant with time. The only variables in the. equation are R and R and they appear in the same power and increase with time in similar proportions to produce a balance. If R after 1 sec. is 3 times R then laterwhen it is greater it will be still 3 times the new R We can see by inspection and visualization of the flows;

with time that this balanced condition which develops almost instantly with contact would prevail at later times;

Th situati n abo e th t a e a b an d w h equ faced cylinders is thus demonstrated here with spherical segments or cone-like configurations.

Approximate values of C for diiferent total included angles of the sectors are: 30, .033; 45, .076; 60, .134;

Inspection of the formulae and curves in Figs. 13A, 14A and 15A indicates also that the percentage T is of T can vary very greatly and is related to the relation between the probe T.C. and D. and its cooled path dimensions and the sample T.C. and D and its heated path dimensions The fraction the spherical subtended sector angle of the probe is of a hemisphere determines the ratio between the paths. For many metals the product of specific heatsxdensities are nearly equal. With the others allowance can be made for the diiference by inspection.

By inspection with visualization of the above and by test, it is apparent that a high T.C. and D. and/or wide angle of the probe will be necessary, especially if the T.C. and D. of the sample is fairly high, in order to keep T at a high enough figure to get large enough thermoelectric readings to insure accuracy. By these means choice of a proper hot probe is possible in relation to the T.C. and D.s of the samples especially where two hot probes are used.

Referring now more particularly to the drawings, and

specifically to Figures 1-5 thereof, wherein is illustrated a preferred embodiment of the instant device, 20 gener ally designates a housing or casing, and 21 is a suitable handle on one end of the casing. The casing or housing includes a generally cylindrical body or block 23, preferably fabricated of aluminum or other light relatively good conductivity material and a hollow, externally finned cylinder or insulation section 24, fixedly secure in end to end relation with the body 23. The handle 21 is fixed on the distal end of the cylindrical insulation section 2'4. Of course, the casing may be mounted in a stand, if desired, for moving the apparatus squarely against the sample.

The generally cylindrical body 23, which may be of any relatively high conductivity material, has its distal end provided with a pair of rapidly converging, longitudinally outwardly projecting portions 26 and 27. As illustrated the laterally spaced, longitudinally projecting end portion 26 and 27 are preferably of conical or pyramidal configuration and each tapers or converges at an angle of at least 75.

Viewed otherwise, the probe convergence can be expressed in terms of the cross-sectional area of the probe at, say, inch from the small area contact surface. This area should be at least .15 square inch.

Extending inwards from the distal or outer end of the body 23 are a pair of laterally spaced, parallel bores 28 and 29, each extending coaxially of one body end portion 26 and 27, respectively, so as to truncate the latter. As seen'in Fig. 3, the bores 28 and 29 are provided at their inner end portions with tubular insulators as at 30 and 31, respectively. The heat storage block or body 23 is further formed with a plurality of longitudinally disposed spaced bores, extending through the inner end of the body towards and terminating short of the outer or distal body end. In particular, see Fig. 2, there are provided a main thermostat receiving bore 33, a starting thermostat receiving bore 34, bore for receiving a continuous heater, a control heater receiving bore 36,. a starting heater receiving bore 37, and a thermometer well 38. Accordingly, the bores 33, 34, 35, 36, respectively, receive a main thermostat 39, a starting thermostat 40, continuous heating element 41, a control heating element 42, and a starting heating element 43, while probes orcontact members and 46 are disposed respectively in the bores 23 and 29. The electrical connections for the various thermostats, heating elements and probes will be described hereinafter.

The probes 45 and 46 include, respectively detachable outer end portionsor tips 47 and 48 preferably of truncated configuration to define extensions flush with the conical surfaces of the projections 26 and 27, and threadedly engaged in their associated inner probe members or rods. Stated otherwise, the probe end portions 47 and 48 are of rapidly converging, preferably conical configuration so as to form smooth extensions of their adjacent storage body end projections, and terminate in flat end surfaces 49 and 50 disposed normal to the probe axes. The probe member end surface 49 is disposed inwards longitudinally of the storage body 23, relative to the end surface 50 of the probe 46, so that the latter will hereinafter be called the outer probe while the probe 45 will be termed the inner probe. The detachable probe tips are provided to facilitate economic replacement or sharpening upon wear. The probes may also be of integral or one piece construction and the end surface held at the proper area by repeated sharpening.

While it is preferred to fabricate the outer probe 46, including its detachable end portion 48, of fine silver, silver cadmium oxide or relatively high conductivity material, and the inner probe 45 and its detachable end portion 47 of a lower conductivity silver alloy such as silver cadmium, other materials also may be satisfactorily employed. The qualities found desirable in probe materials are resistance to oxidation or corrosion, a relatively low modulus of elasticity for softness; and, the outer probe is preferably of high thermal conductivity, while the inner probe is preferably of a different lower thermal conductivity.

The tubular insulators 30 and 31 are of suitable material, such as mica or the like. Extending from the probes 45 and 46, through the insulators 30 and 31 and into the insulating section 24 are a pair of conductors 52 and 53. The main thermostat 39 includes an operating stem 54 which extends longitudinally through the insulation section 24 beyond the outer end closure or wall 55 of the insulation section, and is there provided with a manually actuatable adjustment knob 56. The insulation section 24 has its interior surfaces preferably lined with mica or other suitable insulating material, as at 57, and is provided with a transverse, internal wall 58, also preferably fabricated of suitable insulating material. Thus, the interior of the insulation section is sub-divided into an inner compartment 59 and an outer compartment 6b, the former of which is advantageously filled with loose insulation 61 to prevent heating of the probe conductors 52 and 53.

The handle 21 is fixed by any suitable means to the outer side of the insulation section end wall 55, and may be provided with signal lamps 68 and 69, and cable for attachment to a source of electric supply. It will be .noted that the conductors 52 and 53 are connected in the outer compartment of the insulation section to a single conductor 63.

The electrical circuitry of the device Figs. 1-4, and additional circuitry necessary for operation, are schematically illustrated in Fig. 5. The probes 45 and 46 are connected through the conductors 52, 53 and 63 to one side of a five position, two circuit rotary lever switch, generally designated 64 having a plurality of resistances. On the other side of the switch 64, connected thereto by a conductor 73, is a contact member 74, which may be a probe, clamp, plate, etc. A galvanometer, meter or other suitable indicating means, 66 and an adjusting potentiometer 67, are connected to the contact terminals of switch 64; and, as may be understood by inspection, movement of the switch to its different positions serves to vary the galvanometer sensitivity and change its polarity direction reading across the probe 46 and contact member 74. The potentiometer or voltage divider 67 is provided to properly adjust the galvanometer circuit to standardize the scale readings of all machines.

As stated hereinbefore, the main thermostat 39, starting thermostat 40,- continuous heating element 41, controlling heating element '42, and starting heating element '43 are all mounted in the heat storage section of the casing 20. As shown in Fig. 5, each of the heating elements has one end connected to a conductor 70 to one side of an alternating current source 71, and has its other end connected through a conductor 72 to the other side of the source. More specifically, the controlling and starting heating elements 42 and 43 are connected through the main and starting thermostats 39 and 49, respectively, to the conductor 72. Further, the lamps 68 and 69 are connected, respectively, across the controlling and starting heating elements.

In operation, see Fig. 5, with the heat storage body or block 23' relatively cold or unheated both the starting 40 and main thermostats 39, will be closed to permit energization of both the starting heater 43 and control heater 42.. Of course, the continuous heating element 41 is always energized when the circuit is connected to a source of supply. The starter thermostat is fixed to cut out the starting heater at a temperature less than the desired operating or control temperature, as determined by ad: justment of the control or main thermostat. Once the heat storage body 23 has reached the control temperature, which in practice is set around 350 F. above room temperature, the control thermostat 39 will close and the continuous heating element 41 serves to supply the bulk of the radiation heat losses. That is, the continuous heating element is selected of a size to almost maintain the; heat storage body at the desired working temperature by replenishing radiation losses; and, the control heater and its thermostat operate on and off to replenish the rest of the radiation losses and the heat lost by conduction. In order to inform an operator of the thermal condition of the heat storage body at any particular time, illumination of the lamp 69 indicates that the starting thermostat is closed and the block temperature has not yet approached the control temperature, while blinking of the lamp 68 indicates that the block is in the region of the control temperature and being maintained within practical limits. That is, the probes or contact members 45 and 46 are being maintained at a substantially constant temperature relative to the room temperature. The use of a thermostat to automatically supply heat as needed in order to maintain a constant temperature difference between hot and room temperature members is very important, as variable use of the device or variable radiation losses preclude obtaining accurate results with a fixed amount of heat and ambient temperature of the hot member. The differential is determined by calibration of the control thermostat 39 for the desired differential in terms of room temperature so that with change of, room temperature as indicated by a thermometer a new setting can be made. The proper temperature differential may'also be obtained by adjusting the control thermostat until the reading obtained with a known sample corresponds to that previously obtained with the proper temperature. Of course, other temperature sensing controllers can also be used, if desired.

With a metal specimen 75, to be identified, resting on the. cold junction contact member 74, both the specimen and cold junction member being at room temperature, the outer or longer probe 46 is disposed with its end surface 50 in engagement with the specimen. The heated probe immediately conducts heat to the specimen so that the juncture therebetween is at a higher temperature than the juncture of the specimen and contact member 74. Thus, the probes 46, specimen 75 and contact member 74 form a thermoelectric circuit wherein the probe and specimen define the hot junction and the specimen and contact member define the cold junction. The galvanometer 66, of the single polarity direction type for maximum accuracy, is connected through the switch 64 across the thermoelectric circuit to indicate the thermoelectric voltage in the circuit. The switch 64 may be shifted toprovide the galvanometer polarity necessary to. obtain a reading, and also to provide the optimum galvanorneter sensitivity. As the galvanometer readings may be listed for the different metals, by previous tests of known specimens, the pointer indication by reference to the listing will determine the composition of the specimen. Certain readings may be indicative of two and sometimes more metals. However, such an ambiguity may be resolved by contacting the inner, lower conductivity probe 45 with the specimen. This will produce an entirely different reading, which when observed in terms of a listing for that probe will in all probability identify the specimen, if the difiusivities and thermoelectric effects are not too close.

As mentioned hereinbefore, the instant invention affords a steadiness of reading with time, and hence reproducibility of reading, which was not heretofore 0btainable. One element essential to steady readings, as demonstrated by the approximate formula developed hereinbefore, is that the probe be of an approximate conical configuration. Other elements favorably affecting reproducibility of readings, and rapidity of taking repeatable, successive readings are those of (1) a wide probe angle of, convergence, (2) high thermal conductivity and diffusivity of the probe, (3) a finite probe end surface of limited area, and (4) a probe material of sufficient softness to insure surface contact under all conditions.

The graph of Fig. 13A shows two temperature gradients of a probe contacting a specimen, at the same time after contact, but with probes of different angles of convergence. It will be observed that T for the wide angle probe is substantially greater than for the narrow angle probe. A high T affords the advantageous results of producing thermoelectric voltages over a greater range to faciliate reading, and the voltages are higher so as to permit direct and more accurate measurement without complicated voltage reading devices. Another, highly advantageous result of the wide pro-be angle and high T is that the probe end surface will return quickly to the control temperature of the heat storage body upon removal from a specimen, so that a series of accurate readings may be taken on different specimens, or on the same specimen, in rapid succession. That is, the wide angle probe not only produces a high T but also provides thermal conduction conditions favorable to rapid replenishing the heat to the end surface. By test, conical, pyramidal or otherwise convergent probes of at least a angle of convergence have been found satisfactory for a variety of samples.

Probes of different thermal conductivity and diffusivity are compared in the chart of Fig. 14A, wherein it is seen that a probe of high T.C. and D. produces a substantially higher T than a probe of low T.C. and D. The higher T produced by higher T.C. and D. of the probe is advantageous for the same reasons as noted above in connection with Fig. 13A; and, the higher T.C. and D. of the probe also favors rapid replenishing of heat to the probe end surface for rapid and accurate readings. it has been found experimentally that probe T.C. and D. at least equal to that of yellow brass is satisfactory for a variety of samples. In the case of the second or alternate probe for resolving ambiguities in order to obtain a maximum separation of the two readings of the sample in question, it is advantageous to use a second pro-be, corresponding to the inner probe 45, of a noticeably lower T.C. and D. even somewhat lower than the preferred minimum T.C. and D for the primary probe.

The advantage of the larger reading separation is greater than the disadvantage of relatively lesser accuracy of reading.

With favorable probe parameters, the temperature of the probe end will be substantially immediately restored to the control temperature of the heat storage body upon separation of the probe and specimen. However, in addition to the probe characteristics noted above, rapid replenishing'of heat to the probe'endsurfaceto restore the latter to the control temperature requires that the end surface be of a limited area. This is so because a probe end surface or contact area above certain limits provides too great a path for heat flow, and substantially reduces the heat in the probe and the distance the heat has to travel to get to the contact area and raise it to control temperature. The reading will be too low. In addition if the specimen is heated unduly by the relatively great quantity of heat flow which can change the cold junction to above room temperature. The reading will be further depressed. If subsequent contacts are made on a hot portion of the sample, the contact temperature will be higher and unsteady. The conditions of excessively large probe end surfaces are illustrated by the curves of Fig. 16A. Such curves show the temperature gradients in this type of probes upon successive contacts with specimens at room temperatures. It will be noted that the successive contact temperatures, with probe end surfaces too large are successively changed, so that repeatable readings may not be rapidly obtained. For accurate results, it has been found satisfactory to maintain the probe end surface area between a minimum of .002 square inch and a maximum of .03 square inch. While these limits produce satisfactory results, it is generally preferable, especially with harder probe materials, to maintain the contact areas close to the minimum. While softer probe materials, even with contact areas up to the maximum, may with slight pressure insure that area contact is made between the probe and sample, relatively hard probes of maximum contact area may engage only one or more high points of the specimen and produce an unreliable reading at an unrepeatable contact temperature. We have found that probe material should have low modulus of elasticity, less than 20 million p.s.i. so that with a given pressure, the material can be strained more to conform with the sample.

In Figure 15A, it will be observed that a sample of relatively high T.C. and D. permits greater heat flow from the probe, and therefore lowers the contact temperature relative to that of a low T.C. and D. sample. Hence, even samples having substantially the same thermoelectric effect or power will produce widely different readings in the instant device if they are of sufficiently different T.C. and D. From this it will be evident that the method and apparatus of the instant invention do not merely measure thermoelectric power or effect, but measure a thermoelectric voltage, the power of which depends upon the many factors mentioned above including the thermoelectric effect.

The above discussed graphs are merely illustrative and do not necessarily apply to particular specimens or probes.

The first step in getting repeatable readings is choosing the proper probe material, shape, and contact area and then resharpening the area as necessary with use. The probe must be maintained at a given number of degrees, T above room or sample temperature during readings. Contacts for readings will have to be made at such intervals as to let the contact surface temperature, T come back completely to block or probe temperature T before another contact is made. A new unheated area at room temperature on the sample must be used for each contact, or T and hence the readings during contacts will not be repeatable.

Samples, each of known chemical analysis and physical condition when read with this method of standardization, will give repeatable readings and make possible a reading record table with one or more columns for each sample depending on how many probes of different T.C. and D. are used. A copy of this table can be referred to by an operator at the same or another plant with a similar machine when testing otherwise unidentifiable samples. Limited tables could be memorized.

The probe material should not only be of high T.C. and D. for obtaining high T but also should have high electrical conductivity without tendency for interfering surface oxidation, and should have a low modulus of elasticity so that with little pressure the small, contact area can be made to conform closer to the sample surface. A primary probe of pure silver or silver cadmium oxide and a secondary one of silver cadmium of lower T.C. and D. make a good construction and vary little in thermoelectric effect.

Study of the formulae and test has indicated that for very thin or small samples the area of the point should be finer to keep the sizes of the heat paths down. For reading materials under thin work-hardened or heat treated surfaces or platings, the area must be more sub stantial so the disturbing effect of the surface will be diluted or reduced. With platings the thermoelectric eifect between the probe and the plating is virtually cancelled by that between the plating and the sample to yield that between the probe and sample.

With a high T.C. and D. point or probe, variations in thermal conductivity of the sample due to hardness or other heat treatment has a lesser proportional efiect on the reading, so that the sample can be distinguished more readily. This is especially true for higher T.C. and D. samples such as many non-ferrous metals.

Another embodiment of the instant invention is shown in Fig. 6, wherein a continuous heating element 411: is connected directly across the supply lines, and a single controller 39b responsive to thermocouple 39a, is interposed between the supply source and a control heating element 42a to maintain the heat storage body 20a at a substantial constant temperature differential above room temperature in substantially the same manner as the control heating element 42 of the first described form. The controller and thermocouple provide automatic differential temperature control, and correspond to the manually adjusted thermostat of the first described form.

A pair of galvanometers 66a and 66b, including adjusting potentiometers 67a and 67b, respectively, are connected through a variable resistance switch 64a to the hot junction member or probe 46a and cold junction member or probe 74a. That is, the galvanometers are connected across the thermoelectric circuit defined by the probes and sample 75a, and the switch 64a is interposed to vary the sensitivity of the galvanometers. It will be observed that the galvanometers are reversely connected, one to read minus and the other plus, and thus function as a relatively large, zero center instrument.

It will now be understood that the instant invention permits of obtaining a value for every metal, dependent upon its chemical composition and physical characteristics, which value is an interrelation between the thermoelectric effect of the metal and its thermal conductivity and diffusivity. By our standardized conditions, as described in detail hereinbefore, this value for each metal may be quickly, easily and accurately obtained, and the specimen thus identified in terms of previously determined values.

With an accurate instrument and knowing the chemical analysis of the sample, a reading may depend upon the T.C. and D. of the material which may be a function of the work hardness or heat treatment hardness and the readings will give a definite clue as to the hardness.

Also, if the basic thermoelectric powers of the probe and sample are known the T.C. and D. of the sample can be deduced approximately from the reading.

From the foregoing, it is seen that the present invention provides a method and apparatus for use in identifying metals, which fully accomplish their intended objects, and which are well adapted to meetpractical conditions of manufacture and use.

Although the present invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is understood that certain changes. andmodifications may be made within the spirit of the invention and scope of the appended claims.

What is claimed is:

1. The method of determining the identity of a metal specimen, which method comprises: maintaining a pair of electrically connected metal members at a substantially constant temperature diiferential, subsequently contacting said members to a specimen at spaced points thereof to produce a thermoelectric voltage, indicating the quantity of said thermoelectric voltage in terms of previously determined thermoelectric voltage of known metals, to thereby identify said specimen by its thermoelectric voltage, maintaining a second pair of electrically connected metal members at said substantially constant temperature differential, the hotter member of said second pair being of a substantially different thermal diffusivity than the hotter member of said first named pair, contacting said second pair of members to said specimen at spaced points thereof to produce a second thermoelectric voltage, and indicating said second thermoelectric voltage in terms of previously determined thermoelectric voltages of said second pair of members and known metals, whereby an ambiguous indication of identity by said first produced thermoelectric voltage is resolved by said second produced thermoelectric voltage.

2. The method according to claim 1, further characterized by maintaining said second pair of metal members at substantially the same temperature differential as said first named pair of members, the thermoelectric power of said second pair of members being of the same order as that of said first named pair of members so that a difierent indication of thermoelectric voltage is produced by said second named pair of members resulting from the difference in contact temperature differential.

3. The method of determining the identity of a metal specimen, which method comprisespmaintaining a pair of electrically connected metal members at a substantially constant temperature diiferential, subsequently contacting said members to a specimen at spaced points thereof to produce a thermoelectric voltage, indicating the quantity of said thermoelectric voltage in terms of previously determined thermoelectric voltage of known metals, to thereby identify said specimen by its thermoelectric voltage, maintaining a second pair of electrically connected metal members at said substantially constant temperature difierential, the hotter member of said second pair being of a substantially dilferent thermal diffusivity and different thermoelectric power than the hotter member of said first named pair, contacting said second pair of members to said specimen at spaced points thereof to produce a second thermoelectric voltage, and indicating said second thermoelectric voltage in terms of previously determined thermoelectric voltages of said second pair of members and known metals, whereby an ambiguous indication of identity by said first produced thermoelectric voltage is resolved by said second produced thermoelectric voltage.

4. In apparatus of the type described, a cold junction member adapted for electrical connection to a metal specimen, a hot junction member of relatively high thermal difiusivity electrically connected to said cold junction member and adapted to be heated relative to the latter; said hot junction member being of rapidly longitudinally converging configuration in its heated region terminating in an end surface adapted for electrical contact with said specimen at a point spaced from said cold junction member, said hot and cold junction members thus being adapted to complete through said specimen a thermoelectric circuit having a single hot junction, said hot junction member upon contacting said specimen serving to substantially instantaneously heat the contacting area of said specimen to a contact temperature, the relatively large volume of said hot junction member adjacent to its contacting surface supplying ample heat to the latter to maintain said contact temperature substantially constant and enable an operator to obtain a reproducible indication of thermoelectric voltage in said circuit; and a second hot junction member of appreciably difierent thermal diffusivity then said first named hot junction member, said second named hot junction member being electrically connected to said cold junction member, said second named hot junction member being engageable with said specimen to obtain a second reproducible indication of thermoelectric voltage different from said first named thermoelectric voltage.

5. A device according to claim 4, wherein said first named hot junction member is of a thermal diffusivity greater than that of yellow brass and said second named hot junction member is of a thermal diffusivity less than that of yellow brass.

6. In apparatus of the type described, a cold junction member adapted for electrical connection to a metal specimen, a hot junction member of relatively high thermal difiusivity electrically connected to said cold junction member and adapted to be heated relative to the latter and contacted with said specimen at a location spaced from said cold junction member, said hot junction member being of rapidly longitudinally converging configuration terminating in an end surface adapted for said contact with said specimen with the angularity of said converging configuration and the area of said surface such as to maintain the hot junction contact temperature substantially constant and enable an operator to obtain a reproducible indication of thermoelectric voltage in said circuit.

7. Apparatus according to claim 6, the angle of convergence of said hot junction member being at least to provide an adequate flow of heat toward said end surface and maintain the latter at a relatively high temperature.

8. Apparatus according to claim 6, wherein the transverse cross-sectional area of said hot junction member one-fourth inch from said end surface is at least .15 square inches.

9. Apparatus according to claim 6, said hot junction member being of a modulus of elasticity less than 20x10 p.s.i., to insure area contact of said end surface with moderate pressure.

10. Apparatus according to claim 6, said end surface being of an area no greater than .03 square inches.

11. Apparatus according to claim 10, the angle of convergence of said hot junction member being at least 75.

References Cited in the file of this patent UNITED STATES PATENTS 2,330,599 Kuehni Sept. 28, 1943 2,342,029 Zubko Feb. 15, 1944 2,750,791 Hanysz et al. June 19, 1956 FOREIGN PATENTS 23,580 Germany Sept. 5, 1883 

